Various types of tools are used in metal-forming applications such as machining, metal cutting, powder compaction, metal engraving, pin stamping, component assembling, and the like. In particular, punches and dies represent types of metal forming tools used to pierce, perforate, and shape metallic and non-metallic workpieces. Cutting tools and inserts represent types of metal forming tools used in machining applications to shape metallic and non-metallic workpieces. Punches and dies are subjected to severe and repeated loading during their operational life. In particular, punches tend to fail during use from catastrophic breakage induced by the significant stresses at the working end of the tool or other mechanisms, such as wear. The demands on metal-forming tools will become more severe with the introduction of workpieces constructed from steels having higher strength to weight ratios, such as ultra-high strength steels (UHSS's), advanced high-strength steels (AHSS's), transformation induced plasticity (TRIP) steels, and martensitic (MART) steels.
Punches are commonly constructed from various grades of tool steel. Conventional tool steels contain metal carbides that develop from a reaction of carbon with alloying metals, such as chromium, vanadium, and tungsten, found in common steel formulations. The metal carbide particles are initially present in bulk tool steel as clumps or aggregates. The carbide morphology, i.e. particle size and distribution, impacts the tool steel's material and mechanical properties, such as fracture toughness, impact resistance and wear resistance. These material and mechanical properties determine the ability of the tool steel to withstand the service conditions encountered by punches and dies in metalworking operations and serve as a guide in material selection for a particular application.
During tool steel manufacture, tool steel ingots or billets are typically hot worked above recrystallization temperature by hot rolling or forging process. When the tool steel is hot worked, segregated metal carbides may align substantially in the direction of work to form what is commonly known as carbide banding. Hot working of tool steel may also align regions enriched in certain segregated alloy components substantially in the direction of work to form what is commonly known as elemental or alloy banding.
The tendency of segregated metal carbides and alloy components to align along the working direction of hot rolled tool steel (i.e., in the rolling direction) in parallel, linear bands is illustrated in the optical micrographs of FIGS. 1 and 1B and a Scanning Electron Microscopy (SEM) micrograph of FIG. 1A. Collectively, the micrographs show images of polished and etched regions of a commercially available M2 tool steel grade bar stock in the hot rolled condition. At a microscopic level, the carbide and alloy bands have a prominent appearance as apparent from FIGS. 1, 1A, and 1B. In particular, the lighter bands visible in FIG. 1A represent higher alloy contents by weight percent and darker bands represent lower alloy contents by weight percent. In the particular case of S7 tool steel grade shown in FIG. 1A, the higher alloy content lighter bands contain 4.18 wt. % Cr and 2.16 wt % Mo while the lower alloy darker bands contain 3.38 wt. % Cr and 1.30 wt. % Mo. FIG. 1B is an optical micrograph of the banding in as-rolled commercial AISI M2 steel following heat treatment and triple tempering. The specimen was cut and polished and then etched with a 3% nital solution. Measurements of interband spacing, that is, measurements from mid-band on one band to mid-band on an adjacent band, indicate an average of approximately 135 μm with a standard deviation of the average of approximately 21 μm. FIG. 2 is an optical micrograph of a powder metallurgical M4 tool steel grade bar stock, which exhibits similar alignment of the metal carbide and alloy bands substantially along the rolling direction as apparent in FIG. 1A.
After hot rolling, the tool steel is fashioned into a blank that preserves the carbide and/or alloy banding. The directionality of the metal carbides in the carbide bands and the segregated alloy components in the alloy bands increases the probability of brittle fracture and wear along that direction. When tool steel blanks are machined to make tools, like punches and dies, the carbide and alloy bands tend to coincide with the primary loading direction along which fracture may occur during subsequent use.
What is needed, therefore, is a tool with a working region formed from steel that does not contain directional carbide and/or alloy bands.